With the world oil reserves projected to be depleted in about 40 years at the current pace of use, emphasis has shifted to alternative sources of liquid fuel. Currently, ethanol produced from [~]20% of corn (Zea mays L.) grain in the United States contributes [~]3.5% of the volume and 2.5% of the energy equivalent of annual gasoline consumption. Cellulosic biomass has the potential to contribute substantially to the biofuels pool. Corn is the single-largest source of crop residue in the United States. An unaltered cell wall is recalcitrant to hydrolytic enzymes required for the conversion of its polysaccharide fraction into simple sugars before fermentation. Attempts at lowering lignin to increase stover digestibility are generally accompanied by a reduction in biomass. The complexity of the cellulose synthase system poses a challenge in increasing its activity through biotechnological means. Exploitation of natural variation may thus be a more productive route to increase the stover cellulose content. In comparison, the objective of reducing or altering hemicellulose for improved ethanol production as well as digestibility of the grain by monogastric animals may be relatively easier to accomplish through transgenic means. Availability of molecular tools for many of the steps in cell wall biosynthesis and modification has opened the heretofore inaccessible biotechnological avenues to alter the wall composition and perhaps structure for increased ethanol production. 10.2135/cropsci2007.05.0299
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